Just one month into its mission to unlock the secrets of the Martian atmosphere, NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft is already providing unexpected discoveries. MAVEN was launched in 2013 and began its science mission on 16 October 2014. Its eccentric orbit takes it lower than any other Mars orbiter, as well as high into the upper reaches of the planet’s atmosphere.

Bruce Jakosky of the University of Colorado Boulder is MAVEN’s principal investigator. Addressing a press conference at AGU’s Fall Meeting in San Francisco on 15 December, he said that MAVEN’s goal is to understand how and why Mars lost most of its atmosphere in the distant past by studying processes happening now.

Ultimately, the MAVEN science team wants to establish a direct chain from the introduction of energy into the upper atmosphere via the solar wind, sunlight, and other phenomena to how that energy changes the atmosphere and allows gases to escape into space. It is too early in the mission to draw any conclusions, but already, Jakosky noted, scientists can put together some of the links in the chain.

Pathway for Mars Atmospheric Loss

The Suprathermal and Thermal Ion Composition (STATIC) instrument is one of three carried aboard MAVEN that were highlighted at the meeting. It quickly discovered a plume of atmospheric gases escaping into space above the Martian pole, according to Jim McFadden of the University of California, Berkeley, who leads the scientists operating the instrument.

A depiction of the Suprathermal and Thermal Ion Composition (STATIC) instrument on MAVEN near the plume of atmospheric gases it discovered. Understanding the mechanisms driving this plume is essential to explaining how Mars lost much of its atmosphere millions of years ago. Image from NASA.

STATIC focuses on the temperature of ions at high altitude. It is, McFadden explained, the first instrument to measure the acceleration process of very cold particles and their heating to achieve escape velocities. Understanding this ongoing process is essential to explaining the loss of Mars’s atmosphere millions of years ago, he said.

Unexpected Particles in the Lower Atmosphere

In the lower Martian atmosphere, gases are well mixed, while in the upper atmosphere, where they escape, the various ions and elements can cluster together. MAVEN’s eccentric orbit allows it to sample the reservoirs of gases at low altitude, the escaping gases in the upper reaches, and the drivers that move the gases.

Scientists need to make all of the measurements as near simultaneously as they can. “We want to find out how the escape depends on the drivers,” said Jasper Halekas of the University of Iowa. Halekas operates one of the key instruments on MAVEN, the Solar Wind Ion Analyzer (SWIA). He told reporters that SWIA had detected a previously unknown population of high-energy ions at low altitudes—less than 200 kilometers—above the Martian surface.

“We did not expect to see a peak of high energy there,” Halekas said. Supersonic hydrogen and helium ions carried by the solar wind are deflected by Mars’s upper atmosphere, he explained. So why is there the spike in high-energy ions below 200 kilometers?

Halekas’s working hypothesis is that in Mars’s upper atmosphere, some of the charged particles streaming in from the Sun pick up a charge and become neutral. They then sink into the lower atmosphere. Once in the lower atmosphere, they interact with particles there and become charged again, registering on the SWIA instrument.

These readings track exactly with those from the solar wind. “It’s dead-on the solar wind,” he said, but recorded at low altitude. This phenomenon will enable scientists to track the solar wind at Mars more easily than it would be at very high altitude.

A Window Into Mars’s Lower Atmosphere

While Hakelas and colleagues study atmospheric effects on Mars that trickle down from space, Paul Mahaffy of NASA’s Goddard Spaceflight Center in Greenbelt, Maryland, leads research into how low atmospheric processes percolate up to high altitudes. Mahaffy directs the Neutral Gas and Ion Mass Spectrometer (NGIMS). He reported that in its first month of operation, NGIMS had discovered structures and variability in a whole suite of atoms and molecules in the lower Martian atmosphere, a region of space that is being studied in detail for the very first time.

In particular, atmospheric waves propagated over the mountainous terrain, and dust storms affect the upper atmosphere. At high altitudes, the atmosphere is extremely thin, so atoms, molecules, and charged particles behave differently from down below, Mahaffy explained. NGIMS is beginning to unlock the details of lower particles’ actions and interactions.

Connecting Links on a Chain

Jakosky noted that all of MAVEN’s instruments are working as planned, and the science team is therefore optimistic that they will in time provide the complete chain of events that brought the Martian atmosphere from its early state to that which is observed today.

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